Process and apparatus to conduct out particles accelerated in an induction accelerator



May 3, 1960 R. WIDERGE 2,935,691 PROCESS AND APPARATUS TO coNDucT OUT PARTICLES ACCELERATED IN AN INDUCTION ACCELERATOR Filed on. 7, 1953 INVENTOR 5M lm'imae BY mu, JW

h is ATTORNEY:

PROCESS AND APPARATUS TO CONDUCT OUT PARTICLES ACCELERATED IN AN INDUCTION ACCELERATOR Rolf Wideriie, Ennetbaden, Switzerland, assignor to Aktiengesellschaft Brown, Boveri & Cie, Baden, Switzerland, a joint-stock company Application October 7, 1953, Serial No. 384,538 Claims priority, application Switzerland October 18, 1952 3 Claims. (Cl. 328-233) In a magnetic induction accelerator, the particles, hereinafter exemplified by reference to electrons, accelerated by the electric rotation field of a periodically variable magnetic acceleration influence, as is known, are conducted through a so-called control field in a circular orbit. To insure the formation of a potential channel in which the electrons can then rotate stably when they execute radial vibrations about the center line of the course, the intensityof the control field has to decrease with growing radius R, but less extensively than the function 1/R.

The electrons, accelerated in such an apparatus, are mostly directed, in order to produce X-rays, towards an anticathode which is located in the trajectory plane of the electrons in the interior of the, accelerationtube. This is done, for example, by means of a so-called expansion or contraction of the electron course, i.e., by an enlargement or reduction of its diameter, depending on whether the anticathode is located outside or inside of the circuit through which the electrons pass during the acceleration. Expansion or contraction is mostly effected by coils which change the relationship between the control field and the acceleration flux. But it is often desired to direct the accelerated electrons not towards an anticathode but, in order to utilize them elsewhere, to conduct them out of the acceleration tube. If an expansion of the electron circuit is carried out for this purpose, then the electrons leave the orbit at all points of the circumference. But this is not desirable in practice; only an electron ray, which leaves the orbit at a very specific point, is useful. In Swiss Patent No. 265,656 means are disclosed for the deformation of the electron course after the acceleration has been executed, the purpose of said means being the creation of a more or less acute salient of the curve of the course at a very specific point, so that the electrons will, as far as possible, leave the course through which they travelled during the acceleration only at this point.

The means specified in the patent are satisfactorily effective to direct the accelerated electrons towards an anticathode. But it has been found that the conducting out of the accelerated electrons at a very specific point of the orbit by the specified means does not result in satisfactory efiiciency because a considerable part of the electrons leave the orbit at other points. This is due to the fact that the specified means deform not only the lowest point of the potential channel (the so-called equilibrium course) in which the electrons travel during the acceleration, but that, to a certain extent, the edges of the potential channel are shifted similarly. The probability that an electron will leave a potential channel at the desired point is therefore not much greater than the probability that it will emerge at any other point of the potential channel.

The present invention eliminates the shortcomings which exist in the known arrangement for conducting the electrons out of the acceleration tube. It is characterized in that, during the expansion of the circuit over a rela-' tively small part of the circumference, the centripetal 2,935,691 Patented May. 3, 196O ice forces, centered in the direction of the electron course, decrease more extensively with growing radius, than the function l/R. That is to say, the centripetal forces decrease according to the funtion 12R", R being the radius and n being greater than unity. This measure produces a radial instability at a very specific point of the orbit,

whereby the potential channel is severed at this point, so-. to-speak.

The invention will be further described and illustrated with reference to the accompanying drawings in which Fig. 1 is a diagrammatic representation of a segment of a potential channel.

Fig. 2 diagrammatically illustrates coils constructed to produce a magnetic field of constant length, and

Fig. 3 diagrammatically illustrates a coil constructed to produce a field whose length decreases with increasing radius.

Referring to Fig. 1a relatively small part of the circumference is enclosed between the radii A and A B is the center course through which the electrons travel during the acceleration. In order to conduct the electrons out after the acceleration has taken place, the centripetal force P is exerted on each electron during the expansion in the enclosed part, wherein according to the invention:

L P -dA Within the enclosed part, the potential channel, built up during the expansion on the remaining part of the orbit by the control field, which lies between the boundaries G and G does not exist. Due to this point of instability, intensive radial vibrations are imparted to the electrons. When these vibrations have reached a sufficiently large amplitude, the electrons leave the potential channel at the same point because the stabilizing forces are missing here.

The process according to the invention therefore makes it possible to conduct out a larger part of the accelerated electrons at a very specific point of the circuit than was possible by the known method.

The invention also relates to an apparatus to perform the said process. The apparatus is characterized in that it contains coils which produce a magnetic field extending perpendicularly to the plane of the circular course, which, compared to the circumference, has a small length A. According to the invention, the coil is constituted so a A: $1? L P -dA 1 that the magnetic flux falling to the surface element A.dR, decreases with growing radius.

Such coils are most advantageously arranged in pairs symmetrically at both sides of the plane of the orbit. According to Fig. 2, the coils 1, 1 can be constructed so that they produce a magnetic field of at least approximately constant length, but with the radius of variable field intensity. The turns of the coils 1, 1' extend radially to each side of the circular path B along which acceleration of the electrons takes place and they also have a non-symmetrical configuration in the direction of the radius of the electron path, i.e. the turns have progressively increasing peripheral lengths in the direction of the radius of the electron path, so that the magnetic field produced by the coils decreases with increasing radius of the electron path. It is also possible to use a type of coil construction according to Fig. 3 which produces an at least approximately homogeneous magnetic field whose length decreases with growing radius. In Fig. 3 the turns of coils 2, 2' also extend radially to each side of the electron path B and they are also of nonsymmetrical configuration in that they converge in the direction of the radius of the electron path so as to produce the desired decrease in magnetic field with increasing radius of the electron path. Thus in both the Fig. 2 and Fig. 3 embodiments, the magnetic field produced by the coils so far as its effect upon the circling electrons is concerned decreases with an increasing radius of the electron path. In Fig. 2, the distance travelled by the electron stream through the magnetic field produced by coils 1, 1 remains constant but the intensity of the magnetic field decreases with an increase in radius of the electron path. In Fig. 3, the intensity. of the magnetic field remains essentially constant in a radially outward direction but the coil turns themselves converge in that direction so that the distance travelled by the electron stream through the magnetic field decreases with an increase in radius of the electron path.

The coils are fed, especially advantageously, by the current used for energizing the expansion coils or by current proceeding at least approximately proportional to this. The coils can therefore simply be connected in series to the expansion coils, using a transformer if desired.

Attention must be paid to the suppression of the alternating current of line frequency which is induced by the acceleration influence in the expansion coils and which, as a result of the magnetic field produced in the coils used, inaccordance with the invention, could disturb the acceleration process especially at the start. The suppression can be carried'out, for example in that an alternating current of line frequency and of suitable size is introduced into a series connection, whose phase is opposite to that of the induced current.

Because the electrons are already brought close to the outer edge of the potential channel by the expansion process, the magnetic field to be used in accordance with the invention can be relatively weak and can still bring about the intended effect. The technical advantage of the apparatus according to the invention can therefore be found particularly in the small input of additional means for an effective conducting out of electrons.

I claim:

1. Apparatus for conducting electrons accelerated in an electron accelerator of the magnetic induction type out .4 of the circular path travelled by the electrons during their acceleration comprising a pair of coils arranged symmetrically at opposite sides of the plane of said circular path and producing a magnetic field perpendicular to the plane of said circular path, said coils overlying only a relatively small part of the circumference of said cir cula-r path, the turns of said coils extendingradially to each side of said circular path and said coil turns being of non-symmetrical configuration in the direction of'the radius of the electron path so thatthe'rnagne'tic field produced by the coils decreases with increasing radius of the electron path.

2. Apparatus as defined in claim 1 wherein the individual turns of said' coils have progressively increasing peripheral lengths in the direction of the radius of the electron path whereby the magnetic field produced by the coils decreases with increasing radius of the electron path.

3. Apparatus asdefined in claim 1 wherein the individual turns of said coils converge in the direction of the radius of the electron path whereby the magnetic field produced by the coils decreases with increasing radius of the electron path.

References Cited inthe file of this patent UNITED STATES PATENTS 2,152,362 Ruska Mar. 28, 1939 2,193,602 Penney Mar. 12, 1941 2,237,651 Bruche Apr. 8, 1941 2,331,788 Baldwin Oct. 1 2, 1943 2,394,070 Kerst Feb. 5, 1946 2,568,456 Malheiros Sept. 18, 1951 2,572,414 Wideroe Oct. 23, 1951 2,574,975 Kallmann Nov. 13, 1951 2,586,494 Wideroe Feb. 19, 1952 2,640,923 Pollock June 2, 1 953 2,669,652 Adams et al. Feb. 16, 1954 2,760,096 Longini Aug. 21, 1956 2,805,351 Niklas et al. Sept. 3, 1957 

